Additional experiments carried out on an iron treated stock ration have shown that this ration also produces sterility in male rats reared on it. In further work with female rats we have found that this ration induces 100 per cent sterility. There is no first litter fertility. We have shown that the iron treatment brings about the formation of a substance which actively opposes vitamin E. This substance may be extracted from the iron treated ration by ether. It has been shown that even in the presence of high storage reserves of vitamin E, females are unable to utilize these reserves in the presence of the antivitamin. After being changed to a basal ration, these storage reserves may become effective after 4 to 6 weeks. It has been suggested that the antivitamin is stored for that time in the tissues or else brings about some change in the reproductive apparatus of the female that prevents it from nourishing implanted fetuses. The significance of these findings is briefly discussed.